[time-nuts] GPS from a window seat

bg at lysator.liu.se bg at lysator.liu.se
Fri Oct 2 05:52:09 UTC 2009


I have seen receivers needing 180 ohms to switch from internal to external
antenna. It was a slight irritation since the "industry" standard is 200
ohms DC load.

    http://gpsnetworking.com/popups/7.htm

    http://www.gpssource.com/upload/1x4SpecSheet006.pdf

--

    Björn

> I used a bias tee with a capacitor block.  I varied the resistor until I
> could see signals coming from the external antenna, the built in patch was
> shielded with aluminum foil and the receiver verified that no signals were
> coming from the internal antenna.  Some receivers needed only a 10K
> resistor, some models needed 220 Ohms.
>
> John  WA4WDL
>
> --------------------------------------------------
> From: "Brian Kirby" <kilodelta4foxmike at gmail.com>
> Sent: Thursday, October 01, 2009 7:57 PM
> To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
> Subject: Re: [time-nuts] GPS from a window seat
>
>> In order to fake out some Garmin units, when using them via splitters on
>> a
>> external antenna, we put 220 ohm resistors from the center of the coax
>> to
>> the sheild.  The splitters we used were capacitive coupled and this work
>> fine for the Garmins.
>>
>> The Garmin units needed to see some sort of DC load, is you wanted to
>> use
>> the external antenna ports.  If you did not pull any current, the unit
>> stayed on its internal antenna.
>>
>> Brian KD4FM
>>
>> Magnus Danielson wrote:
>>> Jim,
>>>
>>> Jim Palfreyman wrote:
>>>> I've done this with two separate GPS units. One was a basic unit with
>>>> no maps - more designed for bushwalking, boating and other direct
>>>> navigation. It worked really well.
>>>
>>> Mmm. Bushwalking is one of your local specialities I gather...
>>>
>>>> Just recently (a few days ago) flying to Perth I used my car-designed
>>>> Navman. It locked easily and I chuckled as it rapidly swept across
>>>> roads and intersection on the ground at 777 km/hr telling me "Go to
>>>> nearest road".
>>>
>>> That must have been one jumpy ride... 777 km/hr offroad.
>>>
>>> Once again I have been forced to invent a DC fake load to handle the
>>> case
>>> where the current sensing of the GPS receiver is set higher than the
>>> hooked in GPS antenna consumes. The shot-from-the-hip solution involves
>>> a
>>> T-connector, a 10 uH SMD coil and a 470 Ohm SMD resistor. Not ideal in
>>> any sense, but hopefull pulls enought (additional 10 mA) while not
>>> mocking too much with the signal. A similar approach has worked before.
>>>
>>> Cheers,
>>> Magnus
>>>
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>>
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